1. Field of the Invention
The present invention relates to an apparatus for manufacturing a porous optical fiber preform.
2. Description of the Related Art
It was customary in the past to manufacture an optical fiber preform by depositing fine glass particles to cover the surface of a core by means of, for example, CVD (Chemical Vapor Deposition), as shown in FIG. 1. Specifically, fine glass particles generated by a combusting burner 32 are blown against the outer circumferential surface of a core 31 arranged within a reaction vessel 30 while rotating the core 31. As a result, the blown fine glass particles are deposited to cover the surface of the core 31 so as to form a porous optical fiber preform 33. Then, the porous optical fiber preform 33 is vitrified so as to obtain a desired transparent optical fiber preform.
The fine glass particles which were not deposited on the core 31 are transferred together with the waste gas after the reaction into a waste gas processing apparatus (not shown) through a discharge pipe 34
In the conventional apparatus described above, the combustion burner 32 is generally arranged to allow the flame-spurting port thereof to face obliquely upward such that the axial direction of the burner 32 makes a predetermined angle with a vertical plane. What should be noted is that the reaction heat within the vessel 30 causes an upward air stream within the vessel 30, with the result that the flame spurted from the burner 32 is turned upward by the upward air stream. The fine glass particles within the flame are moved upward together with the gaseous stream consisting of the air and flame so as to collide against the core 31 and, thus, to be deposited on the tip and outer circumferential surface of the core 31. On the other hand, the excess fine glass particles which were not deposited on the core 31 are further transferred together with the gaseous stream noted above so as to be discharged together with the waste gas to the outside through the discharge pipe 34.
In the conventional apparatus, it is important to take sufficient measures for preventing the waste gas within the reaction vessel 30 and the excess fine glass particles which were not deposited on the core 31 from negatively impacting the fine glass particles deposited on the core 31. It is also important to take sufficient measures for preventing the excess fine glass particles which were not deposited on the core 31 from being deposited on, for example, the inner surface of the reaction vessel 30. To meet these requirements, it is necessary to discharge efficiently the waste gas and the excess fine glass particles which were not deposited on the core 31 to the outside through the discharge pipe 34. Therefore, it is desirable for the discharge pipe 34 to be positioned on the side opposite to the side on which the fine glass particles within the flame are deposited on the core 31 such that an intake port 35 of the pipe 34 is positioned as close to the core 31 as possible.
However, if the intake port 35 is unduly close to the core 31, the gaseous stream within the reaction vessel 30 is disturbed, resulting in failure to achieve an efficient discharge of the waste gas and the excess fine glass particles. As a result, the excess fine glass particles partly collide against the lower end portion of the intake port 35, giving rise to deposition of fine glass particles D on the lower end portion of the intake port 35.
If the deposition of the fine glass particles D on the lower end portion of the intake port 35 of the discharge pipe 34 grows, the gaseous stream within the reaction vessel 30 is disturbed so as to change the flowing direction of the fine glass particles within the flame. It follows that the fine glass particles fail to collide against the core 31 at a desired position, resulting in failure to obtain the porous optical fiber preform 33 of a desired shape.
It should also be noted that, if the deposition of the fine glass particles D shown in FIG. 1 grows, the grown deposition is brought into contact with the fine glass particles deposited on the core 31 so as to damage to the porous optical fiber preform 33.